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Calculation of the creatinine clearance

OVERVIEW

The creatinine clearance is a widely used test to estimate the glomerular filtration rate (GFR) [1,2]. Creatinine is derived from the metabolism of creatine in skeletal muscle and from dietary meat; it is released into the circulation at a relatively constant rate and has a stable plasma concentration. Like inulin, creatinine is freely filtered across the glomerulus and is neither reabsorbed nor metabolized by the kidney. However, tubular secretion by the organic cation secretory pathways in the proximal tubule accounts for approximately 10 to 20 percent of urinary creatinine in patients with a normal GFR and a progressively higher percentage as the GFR falls [3]. The net effect is progressive overestimation of the GFR with more severe disease. (See 'Increased creatinine secretion' below.)

If the effect of secretion is ignored, then all of the filtered creatinine (equal to the product of the GFR and the serum creatinine concentration [SCr]) will be excreted (equal to product of the urine creatinine concentration [UCr] and the urine flow rate or volume [V]). Thus:

       GFR x SCr  =  UCr x V

       GFR = [UCr x V]/SCr

This formula is called the creatinine clearance and tends to exceed the true GFR by 10 to 20 percent or more depending upon the proportion of urinary creatinine that is derived from tubular secretion [4]. Historically, this error was balanced by an error of almost equal magnitude in the measurement of the serum creatinine. The error in serum creatinine measurement was due to non-creatinine chromogens (such as acetone, ascorbic acid, and pyruvate) that are present in serum and contributed 10 to 20 percent of the creatinine concentration measured by older colorimetric techniques. However, national standardization of serum creatinine assays to creatinine reference materials has largely abolished this error. If a laboratory is using standardized methods, creatinine clearance measurements will consistently be 10 to 20 percent higher than GFR in patients with a normal GFR and progressive higher as the GFR falls. (See 'Increased creatinine secretion' below.)

           

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Literature review current through: Aug 2014. | This topic last updated: Jul 10, 2013.
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